<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    let num = +prompt('请输入一个数')
    let sum = 0
    for (let i = 0; i <= num; i++) {
      sum += i
    }
    document.write(`累加和是${sum}`)
  </script>
</body>

</html>